PhD defence by Moritz Fischer

Title: Single-Photon Emitters in 2D Materials



Principal supervisor: Associate Professor Nicolas Claude Stenger, DTU Fotonik, Denmark
Co-supervisor: Associate Professor Martijn Wubs, DTU Fotonik, Denmark
Co-supervisor: Associate Professor Sanshui Xiao, DTU Fotonik, Denmark


Evaluation Board

Associate Professor Alexander Huck, Department of Physics, DTU, Denmark
Professor Oliver Benson, Humboldt University of Berlin, Germany
Professor Philip Hofmann, Aarhus University, Denmark


Master of the Ceremony

Associate Professor Niels Gregersen, DTU Fotonik, Denmark



Many quantum technologies are based on single-photon emitters, also called quantum emitters. The two-dimensional (2D) material hexagonal boron nitride (hBN) can host quantum emitters that are active even at room temperature. However, the microscopic origin of quantum emitters in hBN is still under debate and their generation mechanism remains elusive. In this PhD project, we address these challenges by generating luminescent centres via a novel process. We choose to start with exfoliated high-quality hBN since this provides an almost pristine material. This almost pristine hBN is irradiated with oxygen atoms and subsequently annealed in nitrogen.


We study luminescent centres under green laser excitation and find several quantum emitters. The luminescent centres are categorised into groups by their photoluminescence line shape. To quantitatively localise luminescent centres in hBN, we develop techniques that allow to determine the density of centres for different irradiation energies and fluences. We find that the irradiation fluence dominates the generation mechanism for Group I and II centres. Since we use almost pristine hBN as a starting material, our collaborators at the Helmholtz-Zentrum Dresden-Rossendorf can model the irradiation process precisely via molecular dynamics simulations. These simulations in combination with systematic experiments clarify the generation mechanism for the first time to our best knowledge. Combining this generation mechanism with theoretical studies in the literature we infer that only two types of defects are responsible for the Group I and II centres. Ab initio calculations of the optical properties of these two defects by our collaborators at DTU Fysik show excellent agreement with experiments.


The external stay of this PhD project allowed to study luminescent centres in hBN at low temperature. Under blue laser excitation, we find two additional groups of luminescent centres that were not found under green excitation at room temperature. Furthermore, we localise one Group I centre under blue excitation at room and low temperature. The low-temperature line shape reveals several phonon sidebands that we assign to individual phonon modes of hBN. In the light of experiments under green and blue excitation, we propose that the excitation and relaxation dynamics of Group I centres can be understood in the frame of the LO-phonon assisted excitation scheme.


Our irradiation-based process uses inexpensive and widely-used equipment to generate luminescent centres in hBN on a large-scale. Moreover, our process could be applied to other gapped 2D materials where it may generate luminescent centres or even quantum emitters. Systematically studying the density of luminescent centres for different irradiation conditions could reveal the generation process and may even provide crucial information in order to infer the microscopic origin of quantum emitters.



tor 03 jun 21
13:30 - 16:30